Real time symmetry modeling
You want to use X-Mirror. It only works on the X-axis but doesn't require a mirror modifier. While transforming and other simple operation work well, more complex operations like deleting or disolving may not be mirrored correctly.

From this answer:


  Sounds like you [want] the X-mirror option.
  
  Open up the tool shelf T switch to the Option tab and
  [check] the X Mirror checkbox.